1. Field of the Invention
The present invention relates to a clamping apparatus adapted to damp an object to be fixed such as a metal mold and a workpiece by a clamp arm of balancing type, and more specifically to a clamping apparatus of the type adapted to drive the clamp arm by a transmission member of an eccentric type.
2. Description of Prior Art
As such damping apparatus there has been known the one, for example as disclosed in the Japanese Patent Laid Open Publication No. 54-36680. As shown in FIG. 24, this apparatus is constituted as follows.
A clamping apparatus 102 extending in the front and rear direction (namely, in the left and right direction in FIG. 24, and the same shall apply hereinafter.) is fixedly secured onto a stationary table 101 of a processing machine, and a metal mold D is placed in front of a housing 103 of the clamping apparatus 102. The metal mold D is adapted to be pressed onto the upper surface of the stationary table 101 by a clamp arm 105. A fulcrum portion 105a is provided in a rear portion of the arm 105, and a driven portion 105b is provided in a midway portion of the arm 105 in the front and rear direction. The fulcrum portion 105a is supported vertically pivotably by the upper surface of a support block 113. A small diameter pin 122 is fitted eccentrically into a large diameter pin 121 fitted into the driven portion 105b, and the opposite end portions of the small diameter pin 122 are fixedly inserted into apertures (not illustrated) of the housing 103. The symbol A designates an axis of the small diameter pin 122, and the symbol B does an axis of the large diameter pin 121.
An upper end portion 119a of the lever 119 is fixedly secured to the large diameter pin 121, and a lower end portion 119b of the lever 119 is connected to a front end portion of a piston rod 150 of a double acting type hydraulic cylinder 106. A clamping actuation chamber 144 and an unclamping actuation chamber 146 are defined before and behind the piston 140 respectively. The symbol 145 designates a spring for holding a clamped condition.
Under an illustrated unclamped condition, while a pressurized oil is discharged from the clamping actuation chamber 144, the pressurized oil is supplied to the unclamping actuation chamber 146. Thereby, the arm 105 is returned to an unclamping position by a return spring 155.
When clamping the metal mold D by the arm 105, the pressurized oil is discharged from the unclamping actuation chamber 146 and the pressurized oil is supplied to the clamping actuation chamber 144 so that the piston 140 and the piston rod 150 are moved rightward. Thereby, the large diameter pin 121 is eccentrically rotated counterclockwise about the axis A of the small diameter pin 122 to strongly swing a clamping portion 105c downward.
Under the above-mentioned clamped condition, as indicated by an alternate long and two short dashes arrow-line in FIG. 24, while a clamping reaction force h acts from the metal mold D to the clamping portion 105c, a fulcrum reaction force f acts from the support block 113 to the fulcrum portion 105a as well as an operation reaction force g acts from the housing 103 to the driven portion 105b through the small diameter pin 122 and the large diameter pin 121. This operation reaction force g is expressed as g=h+f=h.(m+n)/n by balancing vertical forces and balancing moments.
There are, however, following problems associated with the above-mentioned conventional embodiment.
At the end of clamping operation, since the strong operation reaction force g obtained by adding a value of the fulcrum reaction force f to a value of the clamping reaction force h acts on the driven portion 105b, a large friction force acts between fitting surfaces of the clamp arm 105 and the large diameter pin 121 and a large friction force acts also between fitting surfaces of the large diameter pin 121 and the small diameter pin 122.
In order to drive the arm 105 against such large friction forces, it is necessary to manufacture the hydraulic cylinder 106 having a large capacity. As mentioned above, since the operation reaction force g is large, also a force acting on the small diameter pin 122 becomes large. Therefore, in order to receive that large force, it is necessary to increase a thickness of a front wall portion of the housing 103. Accordingly, also a length of the housing 103 in the front and rear direction becomes longer.
As noted above, since driving means such as the hydraulic cylinder 106 is large in capacity and also the length of the housing 103 in the front and rear direction is long, the clamping apparatus 102 is large in size and heavy in weight.